1. Field of the Invention
The present invention is directed to an assembly for effecting filling, withdrawal and fill level control of a cryogenic liquid held in a tank and to a flow control and pressure management system for a cryogenic liquid. More particularly, the present invention is directed to such an assembly and system applied to a vehicle-mounted tank for receiving and holding a cryogenic liquid fuel and for delivering the liquid fuel to the vehicle engine. The liquids intended for transfer by the apparatus and method of this invention exist in a cryogenic state. The present invention is particularly adapted for, but not limited to, a vehicle-mounted tank for efficiently holding liquefied natural gas (LNG), or methane, and a control assembly for efficiently introducing the LNG into the tank and transferring the LNG to the vehicle engine.
2. Description of Related Art
Cryogenic containers that are designed and manufactured for end-use as vehicular fuel tanks used to store extremely cold liquids require a means to fill the container and deliver product from the container. Typically, LNG vehicle fuel tanks are of double wall construction. The inner shell, a pressure vessel containing LNG fuel, is supported within the outer shell. Radiation shielding, such as wraps of polyester sheet aluminized on both sides, is placed in the space between the inner and outer shells, and the space is placed under a high vacuum to provide particularly effective insulation between the inner shell and the ambient. Since LNG is a cryogenic fuel that boils at −258° F. (at normal atmospheric pressure), the pressure vessel support structure must exhibit a very low conductive heat leak. Tank “heat leak” has a dramatic effect on the pressure temperature and density relationships of the LNG thus making it very difficult to control the fuel tank pressure and maintain consistent fuel quality for delivery to the engine. Low heat leak minimizes tank pressure build-up during vehicle non-operational time periods and prevents venting of fuel during a designed “no vent” standby time.